The invention relates to a photographing apparatus, in particular, to a photographing apparatus for use with a microscope which enables a proper correction for the failure of the reciprocity law.
In a photographing apparatus, factors relating to the response of a film used includes film speed (indicated in terms of ASA value or DIN number), film size and the failure of the reciprocity law. A conventional photographing apparatus is capable of accommodating for variations of a film speed and film size. However, the failure of the reciprocity law is little corrected or, if corrected, is not properly corrected.
The failure of the reciprocity law occurs when there is very weak light or very strong light where the relative sensitivity of the film is reduced as a departure from Bunsen-Roscoe's reciprocity law. It typically occurs in a flash photography in which an exposure is achieved for a brief time with a strong spark light source, in a photographing of a star or stars over a prolonged exposure period through an astronomical telescope or in a photographing of a sample under reduced illumination over a prolonged period of time through a microscope. In particular, when photgraphing an object under a very bright illumination or under a very dark illumination to require an exposure period less than 1/1000 second or greater than one-half second, the reaction of the film is not proportional to the product of the illuminance and the exposure period or the amount of the exposure, preventing a photograph of a proper density from being obtained. The behaviour of the failure of the reciprocity law depends on the kind of films. Representing an exposure period in which the failure of the reciprocity law is corrected for by T'.sub.e and an exposure period in a region where the reciprocity law applies (namely, where the photochemical reaction of the film is proportional to the amount of exposure, such region being hereafter referred to as a linear region) by T.sub.e, the following equality applies: EQU T'.sub.e =.alpha.T.sub.e.sup..beta. ( 1)
In this equation, both .alpha. and .beta. are constants which depend on the kind of the film.
Several apparatus have heretofore been proposed which provides a correction for the failure of the reciprocity law. By way of example, Japanese Published Unexamined Utility Model Application No. 33,234/1977 discloses an electrical shutter in which a shutter controlling, electromagnetic mechanism is operated by a CR time constant circuit, formed by a first photoconductor and a capacitor, and which is connected in series with a transistor. The series combination of the time constant circuit and the transistor is shunted by a compensation circuit including a second photoconductor and a resistor. When the second photoconductor exhibits a high resistance, namely, when an object being photographed is under low illumination, the transistor exhibits an increased internal resistance through which a discharge occurs, thereby increasing an exposure period. However, the degree of correction achieved in the electrical shutter is uniquely determined by the illuminance of the object, the internal resistance of the second photoconductor, the capacitance of the capacitor, the supply voltage and the trigger voltage, and hence suffers from a disadvantage that a correction cannot be made in a manner corresponding to the kind of the film used. Japanese Published Unexamined Patent Application No. 86,332/1977 discloses a digital electrical shutter including a counter circuit for storing pulses from an object being photographed and in which an oscillation frequency is automatically decreased in response to a low illuminance to provide a correction for the failure of the reciprocity law. However, in this arrangement, the shutter is of a memory type in which an exposure period is determined by the amount of light prevailing immediately before a photographing operation takes place. Hence, a correction cannot be made for the failure of the reciprocity law in a manner which faithfully follows a change in the illuminance of the object occurring during the exposure process. In addition, the degree of correction is uniquely determined by circuit parameters, and cannot be made in accordance with the kind of the film.
Thus, conventional electrical shutters which are designed to provide a correction for the failure of the reciprocity law are incapable of providing a satisfactory correction in accordance with the film used, and thus cannot satisfy the requirement defined by the equation (1).